The Sea of Monsters and the rest of the Percy Jackson and the Olympians series are set in a modern-day America surrounded by places and elements from Greek mythology. In The Lightning Thief, Chiron explains to Percy that the Greek gods and Mount Olympus move in accordance with the center of the civilized world. They have occupied Greece and Rome, among other places, and are now stationed in New York City. Mount Olympus hovers above the top of the Empire State Building, and Camp Half Blood resides upstate.

The Sea of Monsters expands upon the western world as the center of the gods. When Greece was the center of Olympus’s world, the Sea of Monsters was located in the Strait of Messina, a narrow waterway between the eastern tip of Sicily and the Calabria region on Italy’s mainland. The strait’s rough currents made passage treacherous and inspired stories of Scylla and Charybdis. In Riordan’s western Greek world, the sea is in the Bermuda Triangle, and the presence of monsters explains the triangle’s choppy waters and stormy conditions. The islands of Circe, the Sirens, and Polyphemus were supposedly located near the Strait of Messina, and in The Sea of Monsters, reside within the sea’s new geographic location.